Present evidence indicates that T antigen is the only virally coated protein produced in transformed cells. Hence, it follows that the binding or enzymatic activities associated with T antigen must be important in the conversion of the normal cells to the transformed state. During the past two years we have concentrated on the purification of T antigen as a prelude toward work on the biological function of T. Using nuclei prepared from a transformed SV80 cell line, we have developed a purification procedure which leads to the isolation of a highly purified T antigen. The purified preparation shows only two bands of SDS polyacrylamide gel electrophoresis. The major and minor bands have approximate molecular weights of 85,000 and 94,000 daltons respectively. Both crude fractions and the highly purified fractions of T will stimulate the incorporation of tritium labeled thyomidine triphosphate into DNA when the T antigen is added to lysed nuclei. In addition to T antigen, the TTP incorporation system requires the other deoxyribonucleotide triphosphates and ATP. We plan to characterize this T antigen stimulated DNA synthesis with the goal establishing whether or not T antigen promotes DNA synthesis at new initiation sites.